Reinforcing textile structure for composite materials

ABSTRACT

A reinforcing textile complex for composite materials, comprising a stack of textile layers with a view to their impregnation by a polymer resin, the including complex comprising: an assembly of weft threads ( 2 ); an assembly of warp threads ( 3, 4 ) associated in pairs, each pair comprising two threads of different type, at least one of which ( 3 ) is based on high toughness threads, the two threads of a given pair being woven with the weft threads in a “leno” weave.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage application, filed under 35 U.S.C.§371, of International Application No. PCT/FR2015/050520, filed Mar. 4,2015, which claims priority to French Application No. 1451734, filedMar. 4, 2014, the contents of both of which as are hereby incorporatedby reference in their entirety.

BACKGROUND

Technical Field

The invention relates to the textile industry, and more particularly totextiles used as a reinforcement for composite materials. It morespecifically aims at a complex formed by a stack of reinforcing layersto be impregnated by a polymer resin, especially a thermosetting resin.

It more specifically relates to a configuration of this type of complexenabling one of its layers to have a function both of mechanicalreinforcement and of drainage of the impregnation resin for closedmolds.

Description of Related Art

Generally, the manufacturing of composite materials based on fibrousreinforcements may be performed by infusion techniques, where the resinis introduced into a mold at specific points, and displaces within oraround the fibrous layers towards suction points.

The infusion method is based on three fundamental physical principles,which are pressure difference, resin viscosity, and permeability.Indeed, the resin migration through the textile structure (impregnation)cannot occur if the permeability is not sufficient and if the pressurein the mold is constant.

The permeability of a reinforcement designates its ability to be crossedby a fluid, in the case in point, resin. At a microscopic scale, it islinked to the microporosities of the strands (fiber assemblies). At amesoscopic scale, it is linked to the spaces which separate the strandsforming the reinforcement weave. At a macroscopic scale, it depends onthe reinforcement weave. The permeability is expressed in m².

Conventional fabrics of roving type (twill weave, canvas, standard gauze. . . ), or Non Crimp Fabrics (NCF) used in the infusion method have apermeability in the range from 10⁻¹⁰ to 10⁻¹¹ m² for glass. Such apermeability is generally not sufficient to guarantee a correct fillingof the part, which generally has a large size. To improve thepermeability, two types of infusion may be used for monolithicstructures.

An infusion with an external draining can thus be performed. In thiscase, the resin flows by means of a strongly permeable drainage fabricplaced above the stack of preformed fibers. The pressure differencebetween the resin inlet, located at the draining level, and the vent,located on the base of the preform, causes the infusion of the resin,first in the drainage fabric, and then across the thickness of the drypreforms. The external draining fabric is then removed from the part bymeans of a peel ply. The main disadvantage of this method is the largeamount of waste (peel ply, external drainage net) and the time necessaryto install the consumables.

A method of infusion with an internal drainage is also known. In orderto limit waste, the drainage fabric is positioned within the textilestructure. It is a very porous layer allowing a good resin flow throughthe preform. It generally is a Continuous Filaments Mat or a syntheticnet which will remain in the room. The major disadvantage of this typeof product is the impact on the mechanical properties due to theincrease in the resin rate in the final laminate.

Thus, the Applicant has described in document EP 0 395 548 a textilestructure formed of a stack of two reinforcing layers, almostexclusively formed of high-tenacity yarns, for example, made of glass,imprisoning between them an aerated layer, formed from relatively thinand wavy synthetic yarns. The central layer, which forms the core of thestack, is thus relatively open-worked and provides a passage for resinbetween the two reinforcing layers, which are much less permeable toresin.

This solution, although it has significant advantages, however has thedisadvantage of generating resin build-up areas which have much poorermechanical properties than the external layers, and this all the more asthe fibrous material which forms the core is not formed of high-tenacityyarns.

An alternative solution dedicated to infusion has been provided by theApplicant in document FR 2870861. This solution, deriving from theformer, uses a polyester knitting as a core.

Another solution has also been provided by the Applicant in document EP0 672 776.

In this solution, the fibrous plies are formed of unidirectionalstructures comprising high-count and high-tenacity yarns. Each of theplies is deformed so that the weft yarns have an inclination which isnot perpendicular to the warp direction. A plurality of such plies isassociated, by combining different inclinations of the reinforcingyarns.

The assembly is formed without inserting core layers to ease the flow.The inclination of the different yarns of the stacked plies enables theresin to flow. Although this solution has the advantage of not includingfibrous materials other than those of the reinforcing layers, it howeverhas the disadvantage of a relatively low permeability to resin in thewarp direction.

It should thus be understood that a compromise has to be made betweenthe mechanical performance of the obtained composites and the resin flowspeed.

BRIEF SUMMARY

The invention thus intends to provide a solution which has both a goodlongitudinal permeability to resin for an easy impregnation during theinfusion process, combined with a high mechanical performance for theobtained composite material.

For this purpose, the invention relates to a textile reinforcingstructure for composite materials, intended to form an intermediatelayer to be integrated in a textile complex formed of a stack of textilelayers with a view to their impregnation by a polymer resin.

The intermediate layer is characterized in that it comprises:

-   -   an assembly of weft yarns;    -   and an assembly of warp yarns associated in pairs, each pair        comprising two yarns of different type, one of which at least is        based on high-tenacity yarns, the two yarns of a same pair being        woven with weft yarns in a leno weave.

In other words, the invention comprises forming an intermediate layerwhich has good mechanical properties, due to the fact that it is made ofhigh-tenacity yarns, and which has a good permeability to resin alongthe warp and/or weft direction. This layer is thus used as a “structuralinternal drain”, thus combining the advantages in terms of permeabilityof a synthetic internal drain and of mechanical characteristics close tothose of a standard reinforcement.

Indeed, the leno configuration with two yarns of different natureresults in that some of these yarns, that is, the high-tenacity yarns,have a limited or even no crimp and define together channels where theresin can easily flow.

The channels are all the better defined as part of the warp yarns, thatis, the high-tenacity yarns, all are on the same side of the weft yarnply. Only the warp yarns of the second type hold the main yarnstogether.

The low crimp of high-tenacity warp yarns is all the more significant asthe tension difference between the two types of warp yarns issignificant. It is also by a lesser extent a function of the countdifference between the two types of yarns. This indeed enables to workwith tension differences on the two types of warp yarns, so that theyarn having the lowest count supports the greatest crimp.

In practice, it is now possible to modulate the reinforcement propertiesof the draining layer by using yarns which are also of high tenacity inthe weft, thus providing a bidirectional reinforcement, both in the warpand in the weft direction However, in certain applications, it may beuseful to only use high-tenacity yarns in the warp direction.

For warp yarns, the yarns of the second type, that is, those having thelowest count, may be of different natures, that is, either organicsynthetic yarns, or high-tenacity yarns similar to the main yarns. Inthis last case, the entire characteristic layer can thus be formed withhigh-tenacity yarns, which may be advantageous for certain compatibilityor heat resistance properties, although the yarn of the second type doesnot take part in the mechanical resistance of the product.

Due to the construction of this layer, the mechanical reinforcementproperties in the warp and weft direction may be very finely adjusted byaccordingly selecting the masses per unit area of the weft and warpyarns.

In the case where the mass per unit area of the weft yarns issubstantially equal to that of the warp yarns, the reinforcement issubstantially balanced. This enables to create channels not only in thewarp direction, but also in the weft direction, which provides asignificant permeability in both directions. However, in the case wherethe permeability only needs to be increased in a single direction, thatis, the warp direction, lower-count weft yarns may be used.

The influence of the binding yarns, that is, the warp yarns of thesecond type, may be all the smaller as the mass per unit area of thewarp yarns of the first type is greater by more than eight or at leastfrom three to four times that of the weft yarns of the second type.

The resin flow capacity may be modulated according to the width of thechannels defined between the main yarns. Thus, in a first case, it maybe provided for the channels between yarns to be of the order ofmagnitude of the width of a yarn. Thus, the gap between the warp (andweft) yarns may be between two and three times the width of one of theseyarns.

It is also possible to provide channels of greater width by providing agap between yarns which is for example greater than four times the widthof a yarn.

In practice, the size of the channels between the warp yarns of highestcount may advantageously be in the range from 0.5 to 3 mm for a goodpermeability in the warp direction. Indeed, below 0.5 mm, the intervalis not sufficient to give way to the resin and, above 3 mm, a phenomenonof interlocking of the reinforcements when vacuum is created can beobserved. It can thus be observed that the textile structures placed oneither side of the draining fabric may clog the channels as vacuum isapplied and cause a drop in the permeability of the product.

The intermediate layer may be associated with one or a plurality ofadditional layers enabling to increase the flow capacity. Preferably,the additional layers are formed from high-tenacity yarns having acomposition identical to that of the reinforcing layers of the complex.It may for example be a veil, or a mat of glass fibers, which by itsbulk eases the flowing of resin during the molding, and improves thedraining effect of the characteristic intermediate layer, with no addedsynthetic material. The use of a glass mat also improves the isotropy ofthe complex, by attenuating the anisotropy induced by the directions ofthe reinforcement yarns of the structural draining layer. It is possibleto add an additional layer on one of the surfaces of the intermediatelayer, or two additional layers, one on each surface of the intermediatelayer, with identical or different compositions from one additionallayer to the other. Of course, this additional layer may itself beformed of a stack of elementary layers if need be.

Such an intermediate layer has significant permeability properties atleast in one direction, combined with high mechanical properties. It canthus be associated by lamination with as many reinforcement layers asnecessary. In stacks of a large number of reinforcing layers, it mayreplace a reinforcing layer, thus gaining the draining effect whilekeeping a high mechanical performance level. The lamination mayconventionally be performed by sewing, gluing or needle punching,possibly by assembly with one or a plurality of overlays.

BRIEF DESCRIPTION OF THE FIGURES

The way to implement the present invention, as well as the resultingadvantages, will better appear from the description of the followingembodiments, in relation with the accompanying drawings.

FIG. 1 is a top view of a textile structure forming the intermediatedraining layer of a complex according to the invention.

FIGS. 2 and 3 are cross-section views respectively along planes II-II′and III-III′ of FIG. 1.

FIG. 4 is a cross-section view of a complex according to the invention,including the intermediate layer of FIG. 1.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

Generally, the draining and structuring intermediate layer, such asillustrated in FIG. 1, comprises weft yarns 2 and warp yarns 3, 4. Weftyarns 2 are arranged parallel to one another and have almost no crimp.Warp yarns 3, 4 are associated in pairs.

The weaving is performed by using a leno weave between the two weftyarns 4 and 3 on the one hand, and weft yarn 2 on the other hand. Thetwo warp yarns 3, 4 are interlocked around the frame.

Due to the tension difference between the two warp yarns, and to theircount difference, a configuration where warp yarns 3 of the first typerest on the ply of weft yarns 2 is obtained. Each warp yarn 4 of thesecond type thus passes under a weft yarn 2 and over warp yarn 3,alternately on one side and the other of the main warp yarn 3 associatedtherewith.

Thus, as illustrated in FIG. 2, the main warp yarns 3 are all arrangedon the same side as the ply of weft yarns 2, and the warp yarns 4 of thesecond type, that is, of lowest count, run from one surface to the otherof the structure with a significant crimp.

Of course, the proportions of the different yarns illustrated in thedrawings are given as an example only, and the various yarns may differin reality from this representation.

It is further possible to modify the number of yarns per length unit, inthe warp direction and in the weft direction to adjust the mechanicalproperties of the future reinforcement as well as the resin flowcapacity.

Different practical embodiments have thus been formed.

EXAMPLE 1

weft yarn 2: a 1,200-tex glass yarn, with a 468-g/m² mass per unit area;

warp yarn 3 of the first type: a 1,200-tex glass yarn, with a 438-g/m²mass per unit area;

warp yarn 4 of the second type: a 28-tex polyester yarn, with a 16-g/m²mass per unit area;

1-mm gap between weft yarns

gap between warp yarns: repeated pattern with two yarns separated by 4mm and then 4 yarns separated by from 0.5 to 0.7 mm.

EXAMPLE 2

weft yarn 2: a 600-tex glass yarn, with a 240-g/m² mass per unit area;

warp yarn 3 of the first type: a 600-tex glass yarn, with a 240-g/m²mass per unit area;

warp yarn 4 of the second type: a 28-tex polyester yarn, with a 20-g/m²mass per unit area;

gap between weft yarns: 1.5 mm (approximately)

gap between warp yarns: 1.5 mm (approximately)

EXAMPLE 3

weft yarn 2: a 600-tex glass yarn, with a 276-g/m² mass per unit area;

warp yarn 3 of the first type: a 1,200-tex glass yarn, with a 280-g/m²mass per unit area;

warp yarn 4 of the second type: a 28-tex polyester yarn, with a 8-g/m²mass per unit area;

In this example, weft glass yarns 2 are thinner, but are arranged with asmaller pitch, to form a gap in the order of one millimeter,corresponding to the width of a weft yarn.

EXAMPLE 4

weft yarn 2: a 600-tex glass yarn, with a 276-g/m² mass per unit area;

warp yarn 3 of the first type: a 600-tex glass yarn, with a 240-g/m²mass per unit area;

weft yarn 4 of the second type: a 28-tex polyester yarn, with a 18-g/m²mass per unit area.

gap between weft yarns: 1 mm

gap between warp yarns: 1.5 mm

The properties of these different examples have been measured incomparison with a reference complex, constructed according to theteachings of patent FR 2870861, comprising two reference reinforcinglayers formed of a 500-g/m² glass fabric, and a reference draining coreformed of a warp knitting based on a 110-dtex polyester yarn, having a110-g/m² general weight.

The performances of these four examples may be summed up in thefollowing table:

Reinforcing construction Front infusion ID permeability Mechanicalproperties * (g/m²) Glass tx Permeability Thickness traction 0° Total 0°90° By volume K0° Laminate σ E weight warp weft (%) (m²) (mm) (MPa)(GPa) Reference 482 236 236 45 6.86 · 10⁻¹¹ 1.72 276.3 17.6 reinforcinglayer Reference draining 110 / / / 7.13 · 10⁻⁹ 2.87 96 7.9 coreReference complex 620 265 240 19 2.10 · 10⁻⁹ 4.39 243 12.4 Example Nr 1916 432 468 39 2.71 · 10⁻⁹ 2.76 Example Nr 2 502 240 240 32 3.43 · 10⁻⁹2.65 234.5 14.3 Example Nr 3 564 288 275 34 2.98 · 10⁻⁹ 3.21 212.8 14.9Example Nr 4 536 240 276 34 3.74 · 10⁻⁹ 2.62 * the mechanical tests arecarried out with an identical standard stack (mat + Product to betested + mat)

Permeability is a physical characteristic which designates the abilityof a material to allow the transfer of fluid through a connectednetwork. Darcy's law enables to link a flow rate to a pressure gradientapplied to the fluid due to a characteristic parameter of the mediumwhich is crossed, that is, permeability k.

Darcy's law can be expressed as:

$k = {\frac{Q}{S} \times \frac{\Delta \; L}{\Delta \; P} \times \eta}$

where:

-   -   k is the permeability (in m²),    -   Q is the flow rate through the test piece (in m³/s),    -   S is the cross-section of the test piece (in m²),    -   η is the dynamic viscosity of the fluid (in Pa·s)    -   ΔP is the pressure drop measured between the ends of the test        piece (in Pa)    -   and ΔL, the length of the test piece

The permeability can be measured along 3 axes. The permeabilityindicated in the above table corresponds to the permeability measured inthe plane of the reinforcement, along the warp direction.

The draining properties of this characteristic layer can be expressed incomplexes used to manufacture composite parts. Such complexes include aplurality of reinforcing layers selected for their mechanicalproperties. Thus, as schematically illustrated in FIG. 4, draining layer1 may be integrated within a stack of a plurality of reinforcing layers11-16 formed by weaving of warp yarns 20 and weft yarns 21, and havingnumbers and orientations determined according to the general mechanicalproperties desired for the final composite part.

There appears from the foregoing that the reinforcement structureaccording to the invention enables to combine structural reinforcementproperties with a good permeability, thus providing a drainingstructural reinforcement.

1-11. (canceled)
 12. A textile reinforcement complex for compositematerials, comprising: a stack of textile layers with a view to theirimpregnation by a polymer resin, the stack comprising an intermediatelayer (1) comprising: an assembly of weft yarns (2); and an assembly ofwarp yarns (3, 4) associated in pairs, each pair comprising two yarns ofdifferent type, one (3) at least of which is based on high-tenacityyarns, the two yarns of a same pair being woven with the weft yarns in a“leno” weave, wherein said intermediate layer (1) has a draining rolewithin the complex.
 13. The complex of claim 12, wherein the weft yarns(2) of the intermediate layer (1) are based on high-tenacity yarns. 14.The complex of claim 12, wherein: the warp yarns (3, 4) comprise warpyarns (3) of a first type and warp yarns (4) of a second type; and thewarp yarns (3) of the first type have a higher count than the warp yarns(4) of the second type.
 15. The complex of claim 14, wherein the warpyarns (4) of the second type are based on organic synthetic yarns. 16.The complex of claim 14, wherein the warp yarns (4) of the second typeare based on high-tenacity yarns.
 17. The complex of claim 12, whereinthe mass per unit area of the weft yarns (2) of the intermediate layeris substantially equal to the mass per unit area of the warp yarns (3,4).
 18. The complex of claim 12, wherein: the warp yarns (3, 4) comprisewarp yarns (3) of a first type and warp yarns (4) of a second type; amass per unit area of the warp yarns (3) of the first type is more thanthree times greater than that of the warp yarns (4) of the second type.19. The complex of claim 12, wherein: the warp yarns (3, 4) comprisewarp yarns (3) of a first type and warp yarns (4) of a second type; amass per unit area of the warp yarns (3) of the first type is more thanfour times greater than that of the warp yarns (4) of the second type.20. The complex of claim 12, wherein the gap between at least one of theweft or the warp yarns is between two and three times a width of asingle weft yarn.
 21. The complex of claim 12, wherein the gap betweenat least one of the weft or warp yarns is greater than four times awidth of a single weft yarn.
 22. The complex of claim 12, furthercomprising at least one layer formed by a mat of fibers, in contact withthe intermediate layer.
 23. The complex of claim 22, wherein the layerformed by the mat of fibers is made of a material identical to that ofthe reinforcement layers.